diff --git a/CHANGELOG.md b/CHANGELOG.md
index e33e5652704..5b7dac9be55 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ We use *breaking :warning:* word for marking changes that are not backward compa
 * [#3095](https://github.com/thanos-io/thanos/pull/3095) Rule: update manager when all rule files are removed.
 * [#3098](https://github.com/thanos-io/thanos/pull/3098) ui: Fix Block Viewer for Compactor and Store
 * [#3105](https://github.com/thanos-io/thanos/pull/3105) Query: Fix overwriting maxSourceResolution when auto downsampling is enabled.
+* [#3010](https://github.com/thanos-io/thanos/pull/3010) Querier: Added a flag to override the default look back delta in promql. The flag should be set to at least 2 times the slowest scrape interval.
 
 ## [v0.15.0-rc.0](https://github.com/thanos-io/thanos/releases/tag/v0.15.0-rc.0) - 2020.08.26
 
diff --git a/cmd/thanos/query.go b/cmd/thanos/query.go
index f967d8538ca..a66435bc4cf 100644
--- a/cmd/thanos/query.go
+++ b/cmd/thanos/query.go
@@ -70,6 +70,8 @@ func registerQuery(m map[string]setupFunc, app *kingpin.Application) {
 	maxConcurrentQueries := cmd.Flag("query.max-concurrent", "Maximum number of queries processed concurrently by query node.").
 		Default("20").Int()
 
+	lookbackDelta := cmd.Flag("query.lookback-delta", "The maximum lookback duration for retrieving metrics during expression evaluations. For example with a look back of 7min and a scrape interval of 10min, promql will fill in the gap only for 7min and will show the other 3min as a gap. Should be set to at least 2 times the slowest scrape interval.").Duration()
+
 	maxConcurrentSelects := cmd.Flag("query.max-concurrent-select", "Maximum number of select requests made concurrently per a query.").
 		Default("4").Int()
 
@@ -175,6 +177,7 @@ func registerQuery(m map[string]setupFunc, app *kingpin.Application) {
 			*maxConcurrentQueries,
 			*maxConcurrentSelects,
 			time.Duration(*queryTimeout),
+			*lookbackDelta,
 			time.Duration(*defaultEvaluationInterval),
 			time.Duration(*storeResponseTimeout),
 			*queryReplicaLabels,
@@ -222,6 +225,7 @@ func runQuery(
 	maxConcurrentQueries int,
 	maxConcurrentSelects int,
 	queryTimeout time.Duration,
+	lookbackDelta time.Duration,
 	defaultEvaluationInterval time.Duration,
 	storeResponseTimeout time.Duration,
 	queryReplicaLabels []string,
@@ -312,6 +316,7 @@ func runQuery(
 				NoStepSubqueryIntervalFn: func(rangeMillis int64) int64 {
 					return defaultEvaluationInterval.Milliseconds()
 				},
+				LookbackDelta: lookbackDelta,
 			},
 		)
 	)
diff --git a/docs/components/query.md b/docs/components/query.md
index 1ef59c1f67f..8acfa7c657b 100644
--- a/docs/components/query.md
+++ b/docs/components/query.md
@@ -367,6 +367,14 @@ Flags:
       --query.timeout=2m         Maximum time to process query by query node.
       --query.max-concurrent=20  Maximum number of queries processed
                                  concurrently by query node.
+      --query.lookback-delta=QUERY.LOOKBACK-DELTA
+                                 The maximum lookback duration for retrieving
+                                 metrics during expression evaluations. For
+                                 example with a look back of 7min and a scrape
+                                 interval of 10min, promql will fill in the gap
+                                 only for 7min and will show the other 3min as a
+                                 gap. Should be set to at least 2 times the
+                                 slowest scrape interval.
       --query.max-concurrent-select=4
                                  Maximum number of select requests made
                                  concurrently per a query.
diff --git a/pkg/query/iter.go b/pkg/query/iter.go
index 901aba0f7f3..ff2676dab9f 100644
--- a/pkg/query/iter.go
+++ b/pkg/query/iter.go
@@ -302,7 +302,7 @@ type chunkSeriesIterator struct {
 func newChunkSeriesIterator(cs []chunkenc.Iterator) chunkenc.Iterator {
 	if len(cs) == 0 {
 		// This should not happen. StoreAPI implementations should not send empty results.
-		return errSeriesIterator{}
+		return errSeriesIterator{err: errors.Errorf("store returned an empty result")}
 	}
 	return &chunkSeriesIterator{chunks: cs}
 }
@@ -503,7 +503,8 @@ func (it noopAdjustableSeriesIterator) adjustAtValue(float64) {}
 // Replica 1 counter scrapes: 20    30    40    Nan      -     0     5
 // Replica 2 counter scrapes:    25    35    45     Nan     -     2
 //
-// Now for downsampling purposes we are accounting the resets so our replicas before going to dedup iterator looks like this:
+// Now for downsampling purposes we are accounting the resets(rewriting the samples value)
+// so our replicas before going to dedup iterator looks like this:
 //
 // Replica 1 counter total: 20    30    40   -      -     40     45
 // Replica 2 counter total:    25    35    45    -     -     47
@@ -648,7 +649,7 @@ func (it *dedupSeriesIterator) Seek(t int64) bool {
 	// Don't use underlying Seek, but iterate over next to not miss gaps.
 	for {
 		ts, _ := it.At()
-		if ts > 0 && ts >= t {
+		if ts >= t {
 			return true
 		}
 		if !it.Next() {